Summary
Larger numbers of colonists can be more likely to establish and spread due to the benefits provided by either more individuals (quantity) or a greater diversity of genotypes or phenotypes (genetic diversity). However, the value of higher colonist quantity or genetic diversity varies widely across studies, leaving a great deal of uncertainty in how these respective mechanisms affect colonization success. This variability is potentially driven by differences in which traits are present in respective colonist pools (‘colonist identity’). Studies with high‐performing colonizers (e.g. genotypes pre‐adapted to the colonizing environment) may find increasing quantity or diversity to be beneficial because it increases the chance high‐performers are sampled, while studies with no high‐performers may find no effects of quantity or diversity. Alternatively, quantity and genetic diversity may play little to no role if the smallest populations already contain high‐performing colonists because there is no scope for a sampling effect to operate. We conducted a field mesocosm experiment to determine if variability in the benefits provided by increased quantity or genetic diversity relates to colonist traits. Nine distinct genotypes of Daphnia pulex characterized also by phenotype, were introduced in ‘single’ (one individual) or ‘many’ (nine individuals) introduction quantities and at ‘low’ (monoclonal) and ‘high’ (mixed genotypes) genetic diversities. We found that larger‐bodied D. pulex genotypes benefited less from increased colonist quantity, while increasing genetic diversity tended to have a lower effect on higher growth rate genotypes. Our results show that the trait values of the colonists can determine the benefits gained when colonist quantity or genetic diversity are increased, with potential applications to future research and practical efforts to promote, or prevent, population establishment.
Methodology
Candidate experimental Daphnia pulex genotypes were selected to posses a unique genotype and phenotype to ensure that genotypic differences also corresponded to measurable differences in relevant ecological traits. Genotypes were selected from a pool of 54 D. pulex clones based on a combination of genetic differences determined using twelve microsatellite markers (for detailed genomic methods see Supplementary material Appendix 1) and maximal differences in life-history traits (determined using a pilot experiment; Supplementary material Appendix 2). A total of nine clones with distinct genotypes/phenotypes, referred to as the ‘experimental genotypes’ (Supplementary material Appendix 2 Table A1), were chosen: six from different lakes in central Ontario (the ‘Muskoka’ region) and three from different lakes in southern Ontario in the region surrounding the Queen’s University Biology Station (‘QUBS’).
Life‐history trials
To obtain detailed phenotypic data, we conducted 21‐day life‐history trials on each of the nine experimental genotypes. Life‐history trials were conducted from 31 May 2016 until 26 June 2016. Twenty neonates were individually isolated from each genotype, maintained following the same trial procedures as detailed in Supplementary material Appendix 2, and checked daily for 21 days. Data were collected on age (in days), day of first reproduction, body size (measured from the top to the base of the carapace) at first reproduction, number of offspring released, and daily survivorship. These selected traits differed the most between genotypes and are ecologically relevant to Daphnia colonization. Body size affects predator vulnerability (Pastorok 1981, Riessen and Young 2005), while reproduction and survival affect population establishment and growth. The intrinsic rate of increase (r ) was calculated for each genotype following the life‐history trials by iteratively solving for r in the equation l = Sbx=ae-rxlxmx where lx is the survivorship of 20 D. pulex females from birth to age x, mx is female offspring per female of age x, α is age 0, and β is the maximum age.
Experiment
The field experiment was conducted at the Queen's University Biology Station, ON, Canada from 22 June until 18 August 2016. We employed an unbalanced design in which mesocosms were inoculated by either one D. pulex from one genotype (a ‘single individual/low diversity’ treatment), nine D. pulex of the same genotype (‘many individuals/low diversity’), or a mixture comprised of one D. pulex from each of the nine genotypes (‘many individuals/high diversity’). The single individual/low diversity introductions were replicated six times for each of the nine genotypes (54 mesocosms), the many individuals/low diversity introductions were replicated five times for each of the nine genotypes (45 mesocosms), and the many individuals/high diversity introduction was replicated five times (five mesocosms). Additionally, a control treatment that received no introductions (five mesocosms) was employed to assess natural aerial colonization (109 total mesocosms).